If you use message inspectors to inject custom http headers to RIA Services calls from the Silverlight client, the server response - although valid - results in either an operation exception on the client, or leads to a complete crash of the Silverlight plugin.
This bug only applies to Firefox 3.6.4 or higher, and only if the plug-in isolation feature of Firefox is turned on (it's turned on by default). With Firefox 3.6.3 or lower, the isolation feature turned off, or other browsers, this error does not occur.
The problem does not occur when only plain WCF (no RIA Services) is used to achieve the same.
This bug has been filed in the Firefox Bugzilla database as issue #574171, and apparently forwarded to Microsoft, see here: https://bugzilla.mozilla.org/show_bug.cgi?id=574171